1. Field of the Invention
The present invention relates to a key switch scanning and encoding system useful in an electronic musical instrument.
2. Description of the Prior Art
In an electronic keyboard musical instrument such as an electronic organ or piano, a significant problem relates to interconnection of the keyboard switches and the tone generation electronics. Typically an electronic organ has several manual keyboards, a pedal keyboard and a bank of stop selection switches. if separate wires connect each keyboard and stop switch to the electronics, the result is a maze of wiring which is costly to assemble and difficult to service. Simplication is desired.
One approach is to use time division multiplexing wherein a unique time slot is associated with each keyboard switch. The presence or absence of a pulse in that time slot indicates whether the corresponding key has been depressed. The advantage is that only a single multiplex line need interconnect the keyboards and the instrument electronics. However, there are two shortcomings. First, the time to scan all switches is fixed, since a unique time slot is made available to each switch whether open or closed. As a result, there may be an undesirable time lapse between the depression or release of a key and detection of this new condition. For example, if closure of a certain switch occurs just after that switch was scanned, an entire fixed length scanning cycle will pass before the condition is detected, even if no other switch is closed. Secondly, utilization of the multiplexed signal requires separate decoding circuitry in time lock with the encoder, thereby complicating the requisite circuitry.
A time division multiplex note selection system is shown by Watson in the U.S. Pat. No. 3,610,799. There, the keyboard switches are organized in an array with the twelve note switches in each octave of each manual or pedal keyboard arranged in a separate column associated with that octave and keyboard. All row outputs of the array are combined onto a single multiplex signal line.
The entire array is scanned at a fixed clock rate. Thus during the first twelve clock periods the twelve switches associated with the highest octave of one manual are scanned. Next the twelve switches associated with the second octave of the same manual are scanned, and so forth. With this sequential scanning arrangement, the multiplexed waveform present on the common output line will comprise pulses in time slots corresponding to the notes selected on the keyboard. The total time taken to scan the keyboard array, corresponding to the length of the multiplexed waveform, is Kt, where K is the number of switches in the array and t is the fixed clock period.
To utilize the multiplex signal requires decoder circuitry in time lock with the scanning clock. In the instrument described by Watson, musical tones are generated by repetitive readout of a musical waveshape stored in memory. The phase angle between successive memory sample points establishes the note fundamental frequency. Watson uses the decoded multiplex signal to control this phase angle, and hence to establish the note produced by the instrument. Decoder circuitry derives a phase-angle-controlling signal by comparing the time of occurrence of each pulse on the multiplex line with the contents of a clock-advanced keyboard counter also used to govern scanning of the switching array.
Another time division multiplexing technique is disclosed by Klann in the U.S. Pat. No. 3,614,287 in conjunction with intermanual coupling. By pulse controlled sequential connection of manual coupling and note selection switches, economy of wiring is achieved while enabling the keys on one keyboard to actuate a coupled voice on the same or a different manual.
The commutation of separate waveform generator outputs onto a common line for processing by a single waveform shaper and sound system is disclosed in the U.S. Pat. No. 2,989,885 to Pearson. There, delay line commutation, at a rate which is high in comparison to the generated tones, is used to mix the outputs of key-switch selected waveform generators onto a common line. This technique permits the use of common timbre-determining circuits but requires a separate line from each waveform generator to the associated keyboard switch.
An object of the present invention is to provide a system for keyboard encoding, useful in an electronic musical instrument, which does not require a fixed scan rate independent of how many switches are selected. Open or "OFF" switches are scanned at a higher rate than switches which are "ON". Economy in scanning rate is achieved, so that each newly closed or released switch is detected without significant delay. Encoding signals are derived, indicative of each selected key. The encoded signals are provided via a few lines for direct utilization by electronic tone generation or other circuitry. Economy of wiring is achieved without the limitation of fixed time scanning and without the requirement for time lock decoding at the point of utilization.